The above-identified copending patent applications describe the challenges of developing efficient electric motor drives. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of an energy source to appropriate stator windings, greater functional versatility can be achieved. The use of permanent magnets in conjunction with such windings is advantageous in limiting current consumption.
In a vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. Electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects with other electromagnet members.
Precision controlled performance within brushless motor applications involves the fusion of nonlinear feedforward compensation coupled with current feedback elements. However, feedforward compensation expressions typically rely heavily upon various circuit parameters, such as phase resistance, phase self-inductance and the like, which are depicted illustratively in the equivalent circuit diagram for an individual motor phase in FIG. 1. Vi(t) denotes the per-phase voltage input, R1 denotes the per-phase winding resistance, and Li represents the per-phase self-inductance. Ei(t) represents the opposing back-EMF voltage of the motor per phase and can be approximated by the following expression:Ei=(Keiω)sin(Nrθi)where Kei denotes the per-phase back-EMF coefficient, ω(t) represents the rotor velocity, Nr denotes the number of permanent magnet pairs, and θi(t) represents the relative displacement between the ith phase winding and a rotor reference position.
The voltage Vi(t) may be defined as follows:
                                          V            i                    ⁡                      (            t            )                          =                                            E              i                        ⁡                          (              t              )                                +                                    R              i                        ⁢                                          I                i                            ⁡                              (                t                )                                              +                                    L              i                        ⁢                          ⅆ                              ⅆ                t                                      ⁢                                          I                i                            ⁡                              (                t                )                                                                                                              i          =          1                ,        2        ,        …        ⁢                                  ,                  N          s                    where
Vi(t) is the voltage across the winding;
Ii(t) is the phase current;
Ri is the winding resistance;
Ei(t) is the back-EMF;
Li is the winding self-inductance; and
NS is the number of stator phase windings.
The voltage Vi(t) is supplied by a regulated DC power source with a limited voltage. Since the back-EMF term is proportional to motor speed, there is a limit for the phase current Ii(t) above certain speeds.
Assuming that the magnetic flux distribution in the air gap is sinusoidal, the steady-state behaviors of the back-EMF and phase current may be defined as follows:                Ei(t)=Ei sin(θi(t))=Ke,iω sin(Nrωt+Δi)        Ii(t)=Ii sin(θi(t))=Ii sin(Nrωt+Δi)i=1,2, . . . , NS and the average total torque is        
      T    _    =            1      2        ⁢                  ∑                  i          =          1                          N          s                    ⁢                        K                      τ            ⁢                                                  ⁢            i                          ⁢                  I          i                    where
Nr is the number of PM pole pairs;
Kei is the back-EMF coefficient;
ω is the motor speed;
Δi is an offset angle that depends on motor geometry;
 T is the total average torque output; and
Kei is the torque coefficient.
Hence, the torque output is also limited by power supply constraints. A phase advance control technique has been used to extend speed range operation limited by the maximum power supply voltage. Instead of forming a sinusoidal armature current (or phase current) in phase with the back-EMF, the phase angle of the armature current is advanced with respect to the back-EMF.
For example, U.S. Pat No. 6,373,211 to Henry et al. describes a method for extending speed range operation for a sinusoidally excited permanent magnet motor. The method utilizes the phase advance technique to achieve an extended speed range of operation at reduced phase current. The extended speed range is provided by controlling the phase advance angle a between the current vector and the back-EMF vector. A set of pre-computed tables is used to store different torque values at different speeds. The current phase advance angle is calculated based upon the torque command and sensed speed.
However, the Henry et al. technique does not produce the values of phase advance angle optimized to achieve the maximum torque output with the minimum phase current. Instead, the patent discloses setting the maximum torque Tmax. Thereafter, the speed ω and the required or command torque Tcmd are read. If the command torque Tcmd is greater than the maximum torque Tmax available at that speed ω, then the command torque Tcmd is reduced. The phase advance angle is calculated for that reduced value of the command torque Tcmd.
Hence, the prior art phase advance technique provides the phase advance angle for achieving an extended speed range of operation at reduced phase current. However, the prior art does not teach optimizing the phase advance angle and the amplitude of phase current to minimize power consumption.
In a vehicle drive environment, wherein power availability is limited to an on-board supply, it is highly desirable to attain a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. As described in those applications, electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects occurring from interaction with other electromagnet members.
Hence, the need exists for phase advance optimization to enable a motor to deliver increased torque output at minimum power consumption.
Moreover, a conventional phase advance technique does not provide phase advance optimization for each phase of a multiphase motor. However, due to phenomena affected by mechanical/manufacturing tolerances and other structural characteristics, each motor phase will exhibit a range of values for each circuit element. Factors that can affect the magnitudes of the circuit parameters include: the net flux linkage of the electromagnet core; fluctuations in the inductance of the core with respect to the electrical circuit; variations in the resistance of the phase winding due to changes in manufacturing tolerances such as the cross sectional area and winding tension; variations in the permeability of the core (related to the grade and the processing and finishing history of the material); phase winding technique (uniform or scrambled wound) or the build quality of the coils on each stator core; position of the electromagnet and permanent magnet interaction (i.e., permeance of the magnetic circuit); variations in the air gap flux density, which is dependent on the permanent magnet rotor magnet sub assembly; residual magnetic flux density; biasing magnetic field due to external magnetic fields; shape of coil wire (rectangular, circular or helical); winding factor achieved in the coil; manufacturing tolerances achieved in the core geometry which could alter the cross sectional tolerance of the core; the effective length over which the coil is wound.
Typically, motor control strategies assume uniformity of parameter values over the entire motor. One median parameter value is taken to represent all corresponding circuit elements of the motor. This lumped parameter approach often leads to degradation in tracking performance due to over/under compensation of the control strategy due to parameter value mismatch within individual phase compensation routines. Such assumed parameters are prone to even greater discrepancies with stator structures configured as autonomous ferromagnetically isolated core components.
Thus, the need exists for a phase advance optimization technique that produces the optimum phase advance angle and optimum amplitude of phase current to maximize the motor output torque at minimum power consumption, and accounts for the parameter variations in the separate phase windings and stator phase component structures.